The present invention is generally directed to an apparatus and method for implanting an anterior installed intervertebral fusion cage system which can be selectively expanded anteriorly between two adjacent vertebrae to cause them to change position relative to each other and produce a normal alignment of the spine, while promoting fusion of the vertebrae. More particularly, the invention discloses an apparatus and method for surgically positioning an implant having a fusion cage and one or more alternative expansion caps which may be intercoupled with the cage to cause expansion of the anterior portion of the cage to form an adjustable wedge for alignment of two adjacent vertebral bodies in accordance with a predetermined and desired spinal curvature.
The implant of the present invention preferably presents an anterior surface which is flush or slightly recessed within the intervertebral joint, so that it does not abrade or otherwise injure surrounding tissues. In certain embodiments the device further includes structure for supporting a substantial portion of the front of the implant against a layer of harder, more compact bone at the anterior surface of the vertebrae in order to reduce the likelihood of subsidence of the device into the bone. Adjacent cages between a pair of vertebrae are preferably linked transversely to provide additional stabilization of the vertebrae.
The spine is a column of stacked vertebrae, each having a rounded, anterior element, or vertebral body which is weight-bearing. The vertebral bodies are separated from each other and cushioned by a series of fibrocartilage pads or discs which impart flexibility to the spine. Aging, injury and disease, such as degenerative disc disease, may result in drying out or collapse of the discs, causing back and leg pain. In some cases the disc or vertebra is damaged beyond repair or must be removed for medical reasons.
While the spinal column appears to be straight when viewed from an anterior or posterior vantage point, when viewed laterally it is apparent that it is actually comprised of four curved regions. In some congenital conditions such as scoliosis and kyphosis, excessive curvature or other displacement of the spinal vertebrae of the spine occurs.
Treatment of weakness, injury or improper curvature by removal of a disc and fusion of adjacent vertebral bodies (arthrodesis) has become relatively commonplace in recent years. More than 20,000 such interbody fusions of the lumbar region alone are now performed annually in the United States. Fusion of adjacent vertebral bodies is generally accomplished by implantation of a cage-like device in the intervertebral space. The cages are apertured, and include a hollow interior chamber which is packed with live bone chips, usually harvested from the patient""s hip, less frequently from the leg, spine or ribs, or bone may be obtained from a bone bank. A bone substitute may also be employed. Following implantation, bone from each of the adjacent vertebrae grows through the apertures to fuse with the bone of the other vertebrae above and below the cage, thus stabilizing the area. The fusion process may take six to twelve months and it is desirable to stabilize both the vertebrae and the cages during the fusion process.
Once the fusion cage has been inserted, the angular orientation of the top and bottom surface of each cage is of importance, because this orientation determines the fixed angular alignment of the facing surfaces of the two vertebrae upon fusion. The cervical and lumbar curves each present a region of normal anterior convexity and posterior concavity or physiological lordosis. There is a need for an implant which can be adjusted in situ to conform to and maintain lordosis of the segments involved in the fusion or adjusted to correct a preexisting deformity and to restore or initiate proper angular vertebral alignment along the spine.
Like most other bones, the bones of the spine and in particular, the vertebral bodies, consist of a core of spongy, cancellous tissue surrounded by a rim of harder, more compact bone. One problem associated with the implantation of intervertebral fusion cages has been eventual subsidence of the cage into the softer or spongier bone that is normally on opposite sides of a disc following implant. However, there is an anterior crescent of harder bone close to the edge of the vertebral bodies. There is a need for an implant which can be installed to provide support along the full length of the upper and lower face of the implant cage, for positioning the cage against a substantial length of the harder, outer rim of bone to provide better anterior support.
Normally, a pair of fusion cage implant devices are inserted into the area previously occupied by a disc in spaced relationship to each other. In order to provide lateral stability, it is desirable to link the two cages together. There is a need for the cages to be adjustable in situ to preserve or restore coronal, axial and sagittal alignment. It is also preferable that the cages be linked by a structure which is recessed within the intervertebral joint. When the cages are inserted into the anterior portion of the intervertebral space, any structure which projects beyond the anterior surface of the vertebral body may cause irritation or damage to the surrounding tissues and vasculature, especially major arteries that are located close to the spine, or to the ligaments and muscles along the spine.
The apparatus and method of the present invention are specifically designed to provide both independent intervertebral implants and transversely linked pairs of implants, which can be selectively expanded anteriorly to conform the vertebrae to a desired angle of curvature of the affected spinal region while supporting the anterior margin of the adjacent vertebral bodies and to do so without abrading or damaging the surrounding tissues subsequent to insertion.
The present invention is directed to an apparatus and method for implanting an intervertebral cage containing a bone graft to allow for the fusing together of adjacent vertebrae, while maintaining or correcting the angular alignment of the spine. The invention provides an improved fusion cage that allows selective adjustment between adjacent vertebrae. The apparatus includes a pair of cage units that have tops and bottoms and are each adjustably coupled to an expansion cap, such that the top and bottom form a wedge which may be adjusted to support the adjacent vertebrae at a predetermined angle. The cage is formed of a resilient material and is generally U-shaped including a pair of legs connected by a rear plate. The expansion cap is urged, normally by a bolt threaded to the rear plate to wedge between and, thus, separate the free or anterior ends of the legs to a desired angular configuration.
The cage unit is fenestrated and hollow so as to receive a packed, harvested bone graft or bone substitute material. Alternatively, the connecting bolt may be fixed to the rear of the cage unit and the cap driven by rotating a nut on the bolt. The cage unit and expansion cap may be configured for self-locking engagement. The expansion cap may also include anterior upper and lower horizontal bone supporting structure and an anterior recess. A pair of adjustable cage units is fixedly intercoupled by a recessed link.
A set of caps is provided with each cap producing a different expansion so that a surgeon may select the cap best suited to provide the desired angular configuration between adjacent vertebrae. The caps are also configured to provide additional end plate support along a substantial portion of the front edge of the vertebral bodies.
The principal objects of the present invention are: to provide an improved method and apparatus for fusing together adjacent vertebrae; to provide such a method and apparatus for implanting an intervertebral fusion cage system for introducing a bone graft between adjacent vertebrae; to provide such a method and apparatus for implanting an intervertebral fusion cage system while maintaining or correcting the angular alignment of the vertebrae of the spine; to provide a method and apparatus for implanting an intervertebral dual cage system; to provide such a method and apparatus for adjustment of the alignment and balance of the spine in situ; to provide such a method and apparatus for especially engaging along a substantial length thereof the anterior, hard and compact bone layers of adjacent vertebral bodies; to provide such an apparatus having an intervertebral cage which is adjustable in situ; to provide such an apparatus having two such independently adjustable intervertebral cages; to provide such an apparatus having two intervertebral cages joined by a fixed link and that can be inserted non-parallel to each other (either in toe in and toe out or skew) and/or biased to provide better purchase to the overall system; to provide such an apparatus having two such intervertebral cages joined by a link which is recessed from the anterior surfaces of the adjacent vertebrae; to provide such an apparatus having a set of expansion caps that each provide a different degree of expansion to allow for variation in the angular configuration between the top and bottom of the cage or alternatively provides a cap that is adjustably coupled with the fusion cage for adjustment of the angle between facing surfaces of two vertebral bodies; to provide such an apparatus having an expansion cap and cage having structure permitting self-locking installation of the expansion cap onto the cage; to provide such an apparatus wherein the cages are round for insertion, but having caps with upper and lower generally linear support regions for engaging the anterior, more compact and hard bone layers of vertebrae; to provide such a fusion cage which includes an interior chamber for supporting a bone graft; to provide such a fusion cage having a group of modular or interchangeable caps with each cap producing a different degree of relative angulation between the top and bottom surfaces of the cage with the caps being usable sequentially and interchangeably to increase the expansion and resulting angulation until the surgeon is satisfied with the result; to provide such a fusion cage which is fenestrated to permit outgrowth of a bone graft into the surrounding vertebrae; to provide such an apparatus having an insertion tool which may be coupled with a fusion cage and uncoupled following insertion of the cage into an intervertebral region; to provide a method for using such an apparatus for implanting a cage unit between two adjacent vertebral bodies, packing the cage unit with a bone graft, coupling the cage unit with an expansion cap for forming the cage unit into a wedge having a predetermined angle associated with each cap between top and bottom surfaces thereof, and permitting the bone graft to grow and fuse the adjacent vertebral bodies together; providing such an apparatus and method which are relatively easy to use, inexpensive to produce and particularly well-suited for their intended usage.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.